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Managed Voice and Data Services Using AAL2

AAL2, referred to as VoAAL2 in a voice network, can integrate the voice and data services offered to the customer. Alternatively, a service provider can begin with an IP-based infrastructure and build out a VoIP call agent architecture to support voice and data services to their customers, which is a more common approach today. Both technologies, VoIP and VoAAL2, offer the value of integrating voice and data while achieving efficient bandwidth use.

This section provides an AAL2 architecture that can provide trunking and integrated access services. By using AAL2, many capabilities can be obtained within the service provider's ATM network:

  • Dynamically change from voice to fax demodulation

  • AAL2 Type 3 cells for reliable dual tone multifrequency (DTMF) relay

  • Dynamically change the compression rate to G.711 for fax calls in midcall

  • Indicate end of speech burst for background noise generation during silence periods at the egress ATM switch

  • Transport up to 248 voice calls with different compression schemes within one or more ATM permanent virtual circuit (PVC)

This architecture provides a Class 4 interconnect replacement, which enables an enterprise to bypass the local Tandem Switch.


A Tandem Switch is a switch that incumbent local exchange carrier (ILEC) networks use to route calls between COs in the same local access and transport area (LATA). These calls are referred to as intraLATA calls. Trunks at each CO are typically interconnected by a Synchronous Optical Network (SONET) ring. The Tandem Switch also connects to an IXC Tandem Office, which is called a point of presence (POP). A POP houses a Class 4 switch that connects into the ILEC's Tandem Switch. The Tandem Switch aggregates interLATA traffic from multiple COs and a trunk facility. An IXC Tandem Office can have dedicated trunks to an ILEC's CO in cases where a high concentration of traffic exists between the CO and the IXC. An IXC handles interLATA traffic.

A Class 5 switch is located in an end office and a Class 4/5 switch is located in a Tandem Office. A Class 5 switch provides local services in the PSTN to the end user. A Class 5 switch provides enhanced calling features, such as call waiting and three-way calling to end users.

The architecture shown in Figure 4-1 depicts a service provider offering integrated access to business customers and trunking service between two PSTN carriers.

Figure 4-1Figure 4-1 Integrated Access and Trunking Service Using AAL2

An end-to-end trunking architecture does not require a call agent. This architecture can reduce the complexity of a mesh of narrowband circuits by having only a single integrated voice and data network. IADs can support the transport of data and voice by using AAL2 and AAL5 from the customer premises to the service provider. The architecture in Figure 4-1 includes IAD 2400 at the business customer site, which terminates into a MGX8000 Voice Gateway at the service provider's network edge. The MGX8000 Voice Gateway adds packet voice capabilities to the MGX 8850 that includes VoIP, VoAAL1, and VoAAL2. The voice signaling from the enterprise customer is tunneled through the MGX8000 Voice Gateway to a Class 4 switch by using AAL2 point-to-point trunking. The data is tunneled through to an ATM switch, such as a Cisco BPX 8600.

Figure 4-1 shows AAL2 trunking services, which are indicated by the vertical dotted line. Multiple T1s or E1s with Primary Rate Interface (PRI) or channel-associated signaling (CAS) terminate from the PSTN to the MGX8000 Voice Gateway. The PSTN cloud represents another service provider offloading its voice traffic to another carrier. The integrated access service uses AAL2 PVCs between two MGX 8850s within the service provider's network. In this application, the MGX8000 Voice Gateway uses an ATM User Service Module (AUSM) card. The IAD 2400 aggregates both voice and data traffic over a T1 access line to the service provider.

CAS and PRI signaling can be supported in this architecture. The CAS information is carried in the AAL2 PVC across the network. CAS is a signaling technique that uses robbed bits within a multiframe, such as a D4 Super Frame (SF) or Extended Superframe. These robbed bits, referred to as ABCD, represent various states and transitions of a voice call. These ABCD bits are transported over the same AAL2 channel as the one used for voice because CAS does not use a separate signaling channel, such as in-band signaling; the CAS bits use AAL2 Type 3 packets as they provide CRC checks for reliability whereas voice traffic uses AAL2 Type 1 packets that are without CRC checks. An important feature that this architecture provides is idle channel suppression. Idle channel suppression stops sending idle channel bits that are generated from the CAS source (for example, a PBX). This mechanism results in significant amounts of bandwidth savings in the service provider's ATM network; this mechanism has no benefit in common channel signaling (CCS) configurations.

The PRI signaling channels, for example, in T1 the 24th time slot and in E1 the 16th time slot, are carried across the ATM network in the AAL5 PVC while the voice traffic, that is the bearer traffic, is carried by the AAL2 PVC. Thus, two different PVCs traverse the end-to-end network. One carries all the data traffic, and the other carries all the signaling traffic. D-channel information is transported across an AAL5 PVC because the signaling is in High-Level Data Link Control (HDLC) format. AAL2 does not support HDLC but AAL5 does.


CCS signaling protocols use HDLC framing, which is a link-layer protocol that provides variable length messages and supports a retransmission error correction capability to ensure 100 percent reliable data delivery. A D channel of an ISDN PRI line uses CCS protocols.

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